UNSEGREGATED OXIDE SUPERCONDUCTOR SILVER COMPOSITE

Abstract: A method for forming unsegregated metal oxide-silver composites includes preparing a precursor alloy comprising silver and precursor elements of a desired metal oxide and oxidizing the alloy under conditions of high oxygen activity selected to permit diffusion of oxygen into silver while significantly restricting the diffusion of the precursor elements into silver, such that oxidation of the precursor elements to the metal oxide occurs before diffusion of the metallic elements into silver. Further processing of the metal oxide composite affords an oxide superconducting composite with a highly unsegregated microstructure. A novel precursor may be used in conjunction with the oxidation under conditions of high oxygen activity to further restrict segregation of metal oxide-silver composite. A precursor composite for preparation of an oxide superconductor includes a primary alloy phase of constituent elements of a desired oxide superconductor; and a secondary phase comprising copper, the secondary phase supported by the primary alloy phase. The composite may additionally include a matrix material for supporting the primary alloy phase and second phase disposed therein. The composite is oxidized to form an oxide superconductor composite.

Description: UNSEGREGATED OXIDE SUPERCONDUCTOR SILVER COMPOSITE Field of the Invention This invention relates to high temperature superconducting oxide composites having unsegregated microstructures. In particular, the invention relates to high temperature oxide superconductor-metal composites prepared by high pressure oxidation of metallic precursors. The present invention further relates to novel precursor materials for the preparation of high Tc oxide superconductors and superconducting composites. Background of the Invention The discovery of high transition temperature superconducting oxides over the past six years triggered an international race to develop high temperature superconducting (HTS) materials. For many applications, in particular electrical power generation, the required HTS materials must operate at high current densities in magnetic fields, and possess adequate robustness, flexibility and critical current tolerance of strain. The stringent performance requirements of the HTS materials has demanded the development of new processing materials and techniques which impart improved superconducting and mechanical properties to the material. Oxide superconductors have been prepared by oxidation of a precursor alloy which contains the constituent metallic elements of the oxide superconductor and the matrix metal (typically, primarily silver). The matrix metal must itself be inert to oxidation or "noble" under the oxidation conditions employed during the process. The heat treatment of the composite is preferably carried out in two steps. A first heat treatment is carried out at relatively low temperatures in order to oxidize the component precursor elements into simple metal oxides or "suboxides". Subsequent heat treatments are then carried out at higher temperatures to convert the suboxide phases into the superconducting oxide phase(s). By "suboxide" as that term is used herein, it is meant simple, binary and/or ternary oxides of the component metals of the superconducting oxide. Despite the relatively rapid diffusion of oxygen through the silver matrix phase, full oxidation of the metallic components into suboxides (in a first thermal treatment) can require extremely long times (for example, 600 hours). During heating at elevated temperatures, the mobilities of the precursor elements and their cationic forms are enhanced. The precursor elements diffuse into the surrounding silver metal, thereby impairing the chemical and physical integrity of the composite. This has the effect of interfering with the formation of oxide superconductor composites with good physical and electrical properties. By "precursor elements", as that term is used herein, it is meant the individual metallic elements of the precursor alloy or their cationic forms. Typically, the precursor elements diffuse as neutral species; however, cations are also expected to contribute, in varying degrees, to the mobility of the precursor elements. Copper has the highest mobility of the constituent precursor elements, but also barium in the yttrium-barium-copper-oxygen (YBCO) system, bismuth and/or lead in the bismuth(lead)-strontium-calcium-copper-oxide (BSCCO) system and thallium and/or lead in the thallium(lead)-strontium-calcium-copper-oxide (TISCCO) system are known to diffuse into the silver matrix. It is expected that given sufficient time and appropriate reaction conditions, other elements will also have measurable mobilities in silver at a level sufficient to impair composite mechanical and electrical properties. In particular, the ability of precursor elements to segregate during thermal treatment has made it difficult to prepare multifilamentary wires having high filament counts. By "multifilamentary wires", as that term is used herein, it is meant wires, rods, tapes and the like, containing oxide superconductor filaments within a matrix metal, where the filaments run axially parallel to one another along the length of the wire, the "longest dimension". By "high filament count", as that term is used herein, it is meant filament densities of greater than 10,000 filaments/cm 2 as determined for a cross-section transverse to the longest dimension. At high filament densities, even the slightest segregation of precursor elements results in the coalescence of individual filaments and the deterioration of wire properties. High oxygen pressure has been used in the internal oxidation of Sn-Ag alloys. Tanaka et al. in U.S. Patent No. 5,078,810 (hereinafter, "Tanaka") observed that high pressure oxidation eliminated scale formation of tin oxides on the outer surface of the silver composite due to the diffusion of tin to the surface. Tanaka addresses the problem of tin migration to the composite surface and not the segregation of tin within the composite. Indeed, segregation such as that observed for complex oxide superconductor composites can not occur in the simple tin oxide-silver composites disclosed by Tanaka. It is an object of the present invention to provide a low temperature oxidation process for preparation of a metal oxide which minimizes diffusion and segregation of the precursor elements into the silver matrix phase without unfavorably affecting the oxidation time. It is a further object of the invention to provide a low temperature oxidation process for preparation of an oxide superconductor which minimizes diffusion and segregation of the precursor elements into the silver matrix phase without unfavorably affecting the oxidation time. It is a further object of the present invention to provide novel composite materials useful in the preparation of oxide superconducting composites. The novel composite of the present invention exhibits reduced segregation of copper into the matrix metal phase and preferential growth of oxide superconductor phase, both of which have a beneficial effect on the superconducting properties of the oxide superconducting composite. It is a futher object of the present invention, to provide a method for preparing composite materials as precursors and intermediates to oxide superconducting composites. It is a further object of the present invention to provide an oxide superconductor composite with improved properties, such as high critical current density. It is another object of the present invention to provide a method for preparing an oxide superconductor composite to improve the superconducting characteristics of the composite. It is yet another object of the invention to provide an oxide superconductor multifilamentary composite wire characterized by a highly unsegregated microstructure and a high filament count. Summary of the Invention The invention provides a method for making an unsegregated oxide superconductor silver composite. The invention also provides a multi-filamentary oxide superconductor-silver composite having an unsegregated microstructure and a high filament count. By "unsegregated" as that term is used herein, it is meant that little or none of the precursor elements have diffused away (or become segregated) from the precursor alloy region. Because diffusion occurs under oxidizing conditions, segregated precursor elements are identified as oxide phases enriched in segregated elements(s), i.e., CuO, PbO, Bi 2 O 3 , etc., in the final metal oxide or oxide superconductor composite. In one aspect of the present invention, an unsegregated metal oxide/silver composite is prepared by forming a precursor alloy comprising silver and precursor elements having the stoichiometry of a desired metal oxide and oxidizing the precursor alloy under conditions of high oxygen activity selected to permit diffusion of oxygen into silver while significantly restricting the diffusion of the precursor elements into silver, so that oxidation of the precursor elements to the desired metal oxide occurs before diffusion of the precursor elements into silver. For the purposes of the invention, "high oxygen activity" is defined as oxygen activity equivalent to the activity of pure oxygen in its gaseous form (C^) at a temperature greater than 200°C and at a pressure greater than ambient. According to the invention, an unsegregated oxide superconductor-silver composite is prepared by further heating the metal oxide-silver composite obtained as described hereinabove under conditions selected to convert the metal oxides into the desired oxide superconductor. In preferred embodiments, the oxidation of the metal precursor to the metal oxide is carried out at a temperature in the range of 250-450 * C, and more preferably 320-430" C. In preferred embodiments, high oxygen activity is attained using high oxygen pressure, oxygen-releasing gases or electromagnetic means. The high oxygen pressure ranges from above ambient to substantially the oxygen threshold pressure for the formation of silver oxide. The P 02 range is preferably 15-3000 psi, more preferably 800-3000 psi and most preferably 1200-1800 psi. In another preferred embodiment, the precursor alloy is oxidized at a temperature in the range of 200 to 450 * C and at an oxygen pressure in the range of 15 to 3000 psi. In yet another preferred embodiment, a second gas is used to dilute the oxygen for enhanced total pressure above the desired oxygen pressure. Total gas pressures can range from 16 to 60,000 psi and the diluting gas may be any non- reactive gas, such as Ar, N 2 , He, Ne, Kr or Xe. Another aspect of the invention provides for a dense (pore or void-free) oxide superconductor composite having a discreet oxide superconductor phase and a silver phase with little or no diffusion of precursor elements into the silver phase. "Little or no diffusion" is defined as having a metal oxide no more than a distance of three microns from the corresponding oxide superconducting phase. An oxide superconductor composite is characterized as having a microstructure in which a cross-section transverse to a longest dimension consists of the silver matrix and a clearly defined oxide superconductor regions having a density of oxide regions of at least 10,000 regions/cm 2 . An oxide superconductor composite is futher characterized as having a relatively unsegregated microstructure, in which a metal oxide phase that results from the oxidation of a segregated precursor component is not observed beyond a distance of 20% of the thickness and width of the oxide superconductor phase. Yet another aspect of the invention provides for a dense (pore or void-free) metal oxide composite having a discreet metal oxide phase and a silver phase with little or no diffusion of precursor elements into the silver phase. "Little or no diffusion" is defined as having a metal oxide no more than a distance of three microns from the corresponding metal oxide phase. A metal oxide composite is characterized as having a microstructure in which a cross-section transverse to a longest dimension consists of the silver matrix and a clearly defined metal oxide regions having a density of oxide regions of at least 10,000 regions/cm 2 . The present invention further relates to novel metal-oxide or oxide superconductor precursors, which may be oxidized under conditions of high oxygen activity. In one aspect of the invention, a composite of the invention includes a primary alloy phase containing constituent elements of a desired oxide superconductor and a secondary phase containing copper. The secondary phase is supported by the primary alloy phase. "Alloy" is used herein in the conventional sense to mean an intimate mixture of phases or solid solution of two or more elements. An alloy can be prepared by milling, cooling from a melt or any other conventional means. In a preferred embodiment, the constituent elements of the primary alloy phase and the copper of the secondary phase, in combination, are present in an amount sufficient to form the desired oxide superconductor. Excess or deficiency of a particular element is defined by comparison to the ideal copper cation stoichiometry of the desired oxide superconductor. In some embodiments, the elements may be present in the stoichiometric proportions of the desired oxide superconductor. In other embodiments, there may be a stoichiometric excess or deficiency of any constituent element to accommodate the processing conditions used to form the desired oxide superconductor. In preferred embodiments, copper is present in stoichiometric excess in the range of 10% to 30% with respect to the ideal copper cation stoichiometry of the desired oxide superconductor. In preferred embodiments, a noble metal may also be present in the primary alloy phase and/or the secondary phase. Noble metals may include, among others, silver, gold, palladium and platinum. In one preferred embodiment, the primary alloy phase supports the secondary phase by disposing the secondary phase within the primary alloy phase. By "disposed within", as that term is used herein, it is meant that the secondary phase is embedded within the matrix material or substantially completely surrounded by the matrix material. The secondary phase preferably is in the form of a wire, rod, foil or particle. In another preferred embodiment, the support is accomplished by contactingly surrounding at least a portion of an outer periphery of the primary alloy phase with the secondary phase. By "contactingly surrounding", as that term is used within, it is meant that at least one surface of the secondary phase is in contact with an outer periphery of the primary alloy phase. The secondary phase preferably is in the form of a wire, rod, foil or particle. In one embodiment of the present invention, substantially all of the constituent element, copper, is in the secondary phase. In another embodiment of the present invention, a portion of the constituent element, copper, is the secondary phase and the balance of the copper needed to form an oxide superconductor is in the primary alloy phase. In another aspect of the present invention, a composite of the invention includes a primary alloy phase containing constituent elements of a desired oxide superconductor, a secondary phase containing copper, the secondary phase supported by the primary alloy phase, and a matrix material for supporting a primary alloy phase and secondary phase disposed therein. By "matrix", as that term is used herein, it is meant a material or homogeneous mixture of materials which supports and/or binds a substance disposed within or around the matrix. In preferred embodiments, the matrix material is preferably a noble metal. The primary alloy phase and the secondary phase may also additionally comprise a noble metal. A noble metal is a material that is inert to chemical reaction and oxidation under the processing conditions used to form an oxide superconductor. Silver is a preferred noble metal. In yet another aspect of the invention, an oxide composite includes a primary oxide phase comprising a sub-oxide of a desired oxide superconductor and a secondary silver phase disposed therein. By "sub-oxide", as that term is used herein, it is meant one or more oxides selected from the group consisting of simple, binary and higher oxides of the constituent elements of a desired oxide superconductor. In yet another aspect of the invention, an oxide composite includes a primary oxide phase comprising a sub-oxide of a desired oxide superconductor, a secondary silver phase disposed therein and a matrix material for supporting a primary oxide phase and secondary silver phase therein. In yet another aspect of the invention, an oxide superconductor composite includes a primary fully dense oxide superconducting phase and a matrix material for supporting a primary superconducting phase. The oxide superconducting phase includes a stoichiometric excess of copper in the range of 10% to 30% with respect to the ideal copper cation stoichiometry of the desired oxide. The composite is characterized by simple oxides of the constituent elements intruding linearly into the matrix. The oxide superconducting phase may include a secondary silver phase disposed within the primary oxide superconducting phase. In another aspect of the invention, an oxide superconductor is prepared. A composite is prepared which includes a primary alloy phase comprising silver and the constituent elements of a desired oxide superconductor and a secondary phase comprising copper. The secondary phase supported by the primary alloy phase. The composite is oxidized under conditions sufficient to form an oxide superconductor. In another aspect of the invention, an oxide superconductor is prepared. A composite is prepared which includes a primary alloy phase comprising silver and the constituent elements of a desired oxide superconductor, a secondary phase comprising copper and a matrix material. The secondary phase is supported by the primary alloy phase and the matrix material supports the primary alloy phase and the secondary phase disposed therein. The composite is oxidized under conditions sufficient to form an oxide superconductor. In yet another aspect of the invention, a metal oxide/silver composite is prepared. A composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor and silver and a secondary phase comprising copper, the secondary phase supported by the primary alloy phase is prepared. The composite is oxidized under conditions sufficient to oxidize the constituent elements of a desired oxide superconductor and under conditions which promote the diffusion of the silver of the primary alloy phase into the region of the secondary phase and under conditions to promote the diffusion of the copper of the secondary phase into the region of the primary alloy phase, so that a pure silver phase occupying substantially the secondary phase is f rmed. In yet another aspect of the present invention, a copper phase-separated composite is prepared. An alloy comprising copper is heated to phase separate copper. The heat treatment includes heating to a temperature in the range of 450 to 700 "C in an inert atmosphere, for a period of about 10 seconds. In yet another aspect of the invention, a textured oxide superconductor is prepared by further texturing a metal oxide phase obtained as described hereinabove by at least one deformation and anneal step. The total deformation strain is in the range of 82% to 89%, and the anneal step is performed under conditions sufficient to form the desired oxide superconductor. In yet another aspect of the invention, an oxide superconductor composite is prepared by oxidizing a composite including a primary alloy phase comprising the consitutuent elements of a desired oxide superconductor and a secondary phase comprising silver disposed within the primary alloy phase. The concentration of silver in the secondary phase promotes preferential growth of the oxide superconductor. The oxidation methods of the present invention, prevent "loss" of the desired metal oxide phase due to diffusion of particular precursor elements into the silver matrix by dramatically reducing precursor element mobilities during processing. The novel precursors of the present invention also prevent segregation in metal oxide composite due to isolation of copper in the secondary phase supported which is supported by the primary alloy phase of constituent elements of a desired oxide. Brief Description of the Drawing In the Drawing: Figure 1 is a plot of diffusivity of oxygen and copper into silver as a function of temperature; Figure 2 is an optical photomicrograph of a multifilamentary YBa 2 Cu 4 O-. oxide superconductor-silver composite oxidized at (a) high temperature and ambient oxygen pressure, and (b) low temperature and high oxygen pressures for the same amount of time; and Figure 3 is a plot of extent of oxidation v. time at several temperatures and for two different pressure regimes. Figure 4 (a)-(d) are cross-sectional views of several embodiments of the composite of the present invention comprising of the secondary phase supported by the primary phase; Figure 5 is a flow diagram of the preparation of the composite of Figure 4; Figure 6 is an optical photomicrograph of a cross-section of a multifilament wire of the invention containing a primary alloy phase, a secondary copper phase and a silver matrix material; Figure 7 is an optical photomicrograph of a cross-section of a multifilament wire of the invention containing a primary oxide phase, a secondary silver phase and a silver matrix phase; Figure 8 is an optical photomicrograph of a cross-section of a metal oxide composite prepared from a precursor composite of the prior art; and Figure 9 is a graph of critical current density v. texturing strain, illustrating an aspect of the method of the present invention. Description of the Preferred Embodiment The present invention provides a method for oxidizing a precursor alloy to optimize the competing effects of oxidation time and precursor element mobility. The precursor elements are immobilized without significantly extending and, in some cases, even reducing the oxidation time. Segregation occurs by migration of the precursor elements of the alloy into the silver matrix, where they are oxidized to form the corresponding metal oxides. Once formed, the cations of the metal oxides can no longer diffuse readily through the silver matrix. However, because the segregated cations are not present in the proper stoichiometry to form the desired metal oxide or oxide superconductor when subjected to subsequent thermal treatments under conditions favorable to the desired oxide formation, the cations remain as particles surrounding the newly formed desired oxide phase (see, Fig. 2). The faster the low temperature thermal treatment is completed, the less time the precursor elements have to diffuse into the silver matrix and the extent of segregation is therefore reduced. However, the mass transport rate of the oxygen through the silver must be increased without altering the transport rates of the precursor elements if significant precursor element segregation is to be avoided. The increased mass transport rate of oxygen can be accomplished by increasing the oxygen activity of the system. Hence, unsegregated metal oxide composites may be achieved by a thermal process at "low" temperatures in combination with conditions of high oxygen activity. At sufficiently low temperatures, the precursor elements of the alloy are effectively prevented from travelling any significant distance through the matrix in the time it takes to fully oxidize the composite. This is possible because the diffusion rate of oxygen through the silver is less dependent upon temperature than the diffusion rate of precursor elements. Fig. 1 illustrates the temperature dependences of copper and oxygen diffusion through silver using a plot of diffusion distance v. temperature. The diffusion distance, x, of the ordinate represents the distance the element of interest has diffused into a silver matrix after a period of 10 hours under ideal conditions. It is clear from Fig. 1, that all diffusion rates increase with increasing temperature and the curve form is exponential. Note that the diffusion of oxygen into silver uses the millimeter (mm) scale for the ordinate, while the diffusion of copper into silver uses the micrometer (μm) scale for the ordinate. However, the diffusion of copper into silver (represented by curve 10) has a more rapidly increasing exponential form than that of oxygen into silver (represented by curve 12). Hence, the distance diffused by copper into silver increases very rapidly as temperature increases, in particular, at a temperature greater than approximately 400 to 430 'C. Hence, diffusion of the precursor elements can be effectively halted by holding the oxidation temperature below the temperature at which diffusion increases at a rapid rate, i.e., less than approximately 450' C. To compensate for the reduced diffusivity of oxygen at the lower oxidation temperatures, the oxygen activity of the system is increased. Diffusivity of the precursor elements of the alloy and oxygen is P 02 independent. However, an increase in oxygen pressure increases the amount of oxygen that is dissolved in the silver matrix and thereby the flux of oxygen through silver is increased. This has the effect of increasing the mass transport of oxygen through silver, while the diffusivity (and mass transport) of the precursor elements is unaffected. In a preferred embodiment, the oxidation temperature range for a precursor alloy is 200°C to 450°C. In particular, precursors to bismuth(lead)-strontium- calcium-copper-containing oxide superconductors have thermal treatment temperatures in the range of 360°C to 430°C. Precursors to thallium(lead)- strontium-calcium-copper-containing oxide superconductors have thermal heat treatment temperatures in the range of 200 * C to 450 ° C. The thermal heat treatment temperature is in the range of 300°C to 350°C for the precursors to rare earth-barium-copper-containing oxide superconductors. The selected temperature ranges described hereinabove have the additional benefit in that they are below the recrystallization temperature of silver (T crysta j) in most instances. The grain structure of silver influences the mass transport of oxygen. If the temperature of oxidation is below the recrystallization temperature of silver ( ~200-400 p C), the silver matrix retains its fine-grained structure (assuming it is fined grained at the onset). It is then possible for oxygen to be transported along the silver grain boundaries. Hence, oxygen transport is enhanced in fine grained silver matrices by oxidation below the recrystallization temperature of silver. The present invention calls for high oxygen activity. The requisite "high oxygen activity" for the present invention is equivalent to the activity of pure oxygen in its gaseous form (Oj) in the temperature range of 250 'C to 450 "C and at pressures in the range of 15 psi to 3000 psi. This can be accomplished in a number of ways, for example by using high oxygen pressure (P 02 ), or by using activated forms of oxygen, generated, for example, by oxygen-releasing gases or electromagnetic means. Oxygen-releasing gases are known in the art and include, but are in no way limited to, ozone, NO x , and the like. Electromagnetic means include high frequency radiation, such as microwave radiation, which are capable of generating reactive forms of oxygen. Anodization is another known way to generate reactive oxygen species. In a preferred embodiment, high oxygen pressure is used as the high oxygen activity means. The rate of oxidation of the precursor elements increases as oxygen pressure increases up to a maximum rate corresponding to the onset of extensive silver oxide formation at the outer surface of the silver composite ("threshold pressure"). Oxidation rates are substantially independent of oxygen pressure beyond this point (threshold pressure of silver oxide formation). The threshold pressure will vary with temperature. For example, the threshold pressure is 1500 psi at 400° C. At lower temperatures, the threshold pressure of silver oxide decreases. For example, a threshold pressure of 300 psi at 330° C is typical. It is desirable but not necessary to operate at oxygen pressures at or near the threshold pressures. It is also possible to operate at oxygen pressures above the threshold pressures. The oxygen pressure preferably is in the range of 15 psi to 3000 psi, more preferably in the range of 800-3000 psi and most preferably in the range of 1200-1800 psi. In another preferred embodiment, the total gas pressure can range up to 60,000 psi. The oxygen pressure will still be in the preferred ranges described above; however, it is diluted with a second gas to enhance the total pressure of the system. The diluting gas may be any non-reactive gas, such as Ar, N 2 , He, Ne, Kr or Xe. The addition of the diluting gas will affect the total oxygen activity and a slightly greater oxygen pressure may be needed in the mixed gas system for oxygen activity comparable to the oxygen-only system. In preferred embodiments, the alloy is oxidized at an oxygen pressure in the range of 800-3000 psi and a total gas pressure of 801-60,000 psi with a second gas used to dilute the oxygen for enhanced total pressure above the desired oxygen pressure. In other embodiments, the alloy may be oxidized at an oxygen pressure in the range of 1200-1800 psi and a total gas pressure of 1201-60,000 psi with a second gas used to dilute the oxygen for enhanced total pressure above the desired oxygen pressure. Enhanced total pressure is useful to prevent local strain/stress splitting of the oxide superconducting grains which may occur due to the volume change associated with oxidation. Typically, the multifilamentary oxide composites of the invention take the form of a silver wire, ribbon or tape. A multifilamentary composite such as that shown in Fig. 2 is prepared in the following manner. The tape is formed by introducing finely divided precursor elements into a silver can and extruding the powder filled can into a wire of much smaller diameter. A number of extruded wires are then grouped together and co-extruded to form a wire having a plurality of metallic precursor filaments therein. Regrouping and co-extruding can be continued until the desired number of filaments are obtained. Tapes having filament counts of 100-2,000,000 have been prepared. The method of the invention is described in the following experiment, with reference to Fig. 2. Figs. 2a and 2b show a cross-sectional view of a YBa 2 Cu 4 O x -silver composite containing multiple filaments of metal oxide in a silver matrix. In the figure, dark areas 20 represent the oxide superconductor phase and light areas 22 represent silver phase. Bundles of six silver-precursor alloy composite multifilamentary tapes are placed in a pressure vessel with a 0.25" (0.635 cm) bore and purged by two "pressurize and drain cycles" at ambient pressure. The tapes are 0.10 " x 0.02" (0.254 cm x 0.0508 cm) in cross section and 1" (2.54 cm) long. They contain 2527 filaments of copper-sheathed YBa 2 alloy filaments with a precursor element stoichiometry of Y 2 Ba 2 Cu 4 and a precursor element fill factor of 10 vol%. Each bundle is heated to 320 "C (at about 2000 psi oxygen pressure) by inserting the pressure vessel into the hot zone of a preheated furnace. The samples are heated for 100 to 200 h, and then are withdrawn from the furnace and air cooled. The samples are rally oxidized after 100 to 200 h. This is more than an order of magnitude faster than the required time for full oxidation at ambient pressure. A polished cross section of the tape is examined by optical microscopy. The fully oxidized sample at 320 'C shows no discemable copper diffusion and the precursor geometry is preserved throughout the composite. Samples processed according to the method described hereinabove have been tested for superconductivity and found to have an onset of zero resistance at a temperature of 82 K. A lead doped Bi-2223 oxide superconductor is processed in the following manner. A lead doped Bi-2223 precursor alloy powder is made by mechanically alloying the elements with up to 50 volume percent silver. The alloy powder is packed into a silver can and deformed repeatedly. A number of extruded wires are then grouped together and co-extruded to form a tape having a plurality of metallic precursor filaments therein. Regrouping and co-extruding can be continued until the desired number of filaments are obtained, yielding the multifilamentary precursor alloy silver matrix composite tape with a typical cross- sectional dimension of 0.75 mm x 3.8 mm. A tape with 259 filaments and a precursor element fill factor equivalent of 15 volume percent is oxidized at 400 * C in 2.9 ksi oxygen for 300 hours for full oxidation with effectively no segregation of the precursor elements from the filaments. Fig. 2 illustrates the improvement in composite microstructure arising from the method of the invention employing both a lower temperature and a higher than ambient oxygen pressure. Fig. 2a is a photomicrograph of a conventionally processed composite cross-section (containing 2527 filaments) that has been oxidized at 600 "C for 200 hours at ambient oxygen pressure. The boundary between the metal oxide regions and the silver matrix is poorly defined due to the high degree of diffusion of metallic elements (primarily Cu as CuO) in the silver matrix. In comparison, Fig. 2b is a photomicrograph of a composite cross-section (containing 2527 filaments) that has been oxidized at 340°C for 200 h in 2000 psi oxygen. The boundary between the circular regions of the metal oxide and the silver matrix are much more clearly defined. The intrusion of the metal oxide into the silver is effectively eliminated in the composite of Fig. 2b subjected to the low temperature/high oxygen pressure thermal treatment of the present invention. Fig. 3 is a plot of the extent of oxidation v. time at two oxygen pressures, ambient and 2000 psi, for Y 1 Ba 2 Cu 4 O.. composite samples. Full oxidation is indicated by an extent of oxidation equal to 1. The rate of oxidation increases by approximately one order of magnitude for an oxygen pressure increase from ambient to 2000 psi, as evidenced by comparison of curve 30 (400 * C, 14.7 psi) to curve 32 (400'C, 2000 psi) in Fig. 3. Although oxidation rate decreases with decreasing temperature, the increase in oxygen pressure can more than compensate for the temperature-induced rate decrease as illustrated by the comparable slopes for curve 30 (400 * C, 14.7 psi) and curve 34 (320 * C, 2000 psi). While a high temperature, high pressure thermal treatment such as that of curve 36 (470 * C, 2000 psi) affords rapid oxidation, the extent of segregation is still considerable and not acceptable for superconducting applications. The resulting microstructure is not optimal because the higher temperature has not permitted an immobilization of the precursor elements. The best improvement in segregation is achieved by lowering the oxidation temperature while employing a high oxygen activity (here, pressure). Once the metal oxide-silver composite is formed, it may be further heat treated using techniques known in the art to prepare an oxide superconductor. For example, the method can be applied to the preparation of all oxide superconductors, including but not limited to, YBa 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K; YBa 2 Cu 4 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K; (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K; (Ba,Pb) 2 Sr 2 CaιCu 2 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K; Hg 1 Ba 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K; Hg 1 Ba 2 Ca 1 Cu 2 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K; wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K; wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K; (Tl, wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K; and (Tl,Pb) ! Sr 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K. Addition of other elements in small quantities to these oxide compositions do not alter the scope or spirit of the invention. The above oxide compositions are nominal; slight deviations therefrom are within the scope of the invention. As the precursor elements are effectively immobilized as cation components of metal oxides, higher temperature processes can be employed without further segregation. For example, the metal oxide-silver composite is heated at 0.075 atm oxygen at 780 * C for 10 h, held at 815 * C for 50 h and then cooled to form the oxide superconductor, Bi 2 Sr 2 Ca 2 Cu 3 O x (2223-BSCCO). Similarly, the metal oxide-silver composite is heated at 15 psi oxygen at 900 "C for 50 h, held at 450 * 0C for 100 h and then cooled to form the oxide superconductor, YBa 2 Cu 3 O x (123-YBCO). The composites of the present invention are characterized by their uniquely unsegregated composite microstructure and high filament count. For example, the silver matrix contains no oxide beyond a distance of 3 μm from the interface of the bulk oxide phase (metal oxide or oxide superconductor) and the silver matrix. Expressed in different terms, the oxide-silver composite has a microstructure in which a cross-section transverse to the longest dimension consists of a silver matrix and a clearly defined oxide (metal oxide or oxide superconductor) region having a density of oxide regions of at least 10,000 regions/cm 2 . Densities of well over 32,000 regions/cm 2 up to 1,500,000 regions/cm 2 have been observed without any degradation of electrical and mechanical properties. In order for densities on this scale to occur without impairment of the electronic and mechanical properties of the composite, little or no segregation must occur between neighboring filaments. The present invention further provides novel oxide superconductor precursors, which are comprised of a primary alloy phase and a secondary copper or silver phase supported by the primary alloy phase. Novel precurors may be used independently with a conventional oxidation method or in conjuction with the high pressure oxidation method described above to obtain an oxide superconductor. The precursor composite can take on many different geometric forms. For example, the primary alloy may be in the shape of a tape or a wire and the secondary form can be wires coaxially aligned within the primary alloy phase as shown in Fig. 4 . (a). A cross-sectional view of a composite 37 transverse to its longest dimension shows aligned copper wires 37a (secondary phase) disposed within a primary alloy phase 37b. The copper wires 37a are coaxially aligned within the composite 37 and are of small dimension. There may be between 1 and 1000 copper wires disposed within the primary alloy phase. The secondary phase may also be in the form of a foil which surrounds an inner wire or tape of primary alloy phase. Fig. 4(b) shows a cross-sectional view of a composite 38 transverse to its longest dimension, in which a copper foil 38a (secondary phase) surrounds a primary alloy phase 37b. The copper foil 38a surrounds at least a portion of an outer surface 38b of the primary alloy phase. The foil 22 is sufficiently contacted to the alloy surface to permit subsequent reaction between the phases, if desired. Alternatively, a foil 38a of the secondary phase could be rolled with a sheet 39b of primary alloy phase 37b to give a helices configuration as shown in Fig. 4 (c). The secondary phase may be in the form of particles disposed throughout the primary alloy phase. Fig. 4 (d) shows a cross-sectional view of a composite 40 which contains copper particles 40a (secondary phase) disposed within a primary alloy phase 37b. The copper particles 40a have a particle size in the range of 100 μm or less and occupy an atomic fraction in the range of 0.05 to 0.65. The atomic fraction is dependent upon the desired oxide superconductor. The primary alloy phase 37b may contain constituent elements of any oxide superconductor. For instance, yttrium (Y) and barium (Ba) for the Y-Ba-Cu-O family of oxide superconductors; bismuth (Bi), lead (Pb), strontium (Sr) and calcium (Ca) for the Bi-Sr-Ca-Cu-O family of oxide superconductors; thallium (Tl), Pb, Sr and Ca for the Tl-Sr-Ca-Cu-O family of oxide superconductors; and mercury (Hg), Pb, Sr and Ca for the Hg-Sr-Ca-Cu-O family of oxide superconductors. It may be preferable to have copper in stoichiometric excess in the range of 10% to 30% with respect to the cation stoichiometry of the desired oxide superconductor. Variations in the proportions and constituents of the elements comprising each oxide superconductor, as are well known in the art, are also within the scope and spirit of the invention. The composite of the present invention can be prepared in any form, such as tapes, rods, ingots or sheets. The secondary phase may be evenly distributed throughout the primary alloy phase so that reaction time for the formation of the oxide superconductor can be minimized. Alternatively, the secondary phase may be grouped in a particular region, such as near the outer periphery of the primary phase or located in the center of the primary phase. It is within the scope of the present invention for the constituent elements of the desired oxide superconductor to contain some, but not all, of the copper which makes up the desired oxide superconductor. The balance of the copper is found in the secondary phase. It is also within the scope of the invention for the alloy matrix 37b to contain additional elements other than the constituent elements of an oxide superconductor. For example, the alloy may contain a noble metal, such as silver. In yet another embodiment, a matrix material supports the primary alloy phase and the secondary phase which is supported therein. It is contemplated that a plurality of primary alloy phase/secondary phase regions can be disposed within the matrix material. Matrix material include, but are in no way limited to, noble metals such as silver, gold, palladium and platinum. The composites of the present invention may be prepared in the following manner. Example L. A primary alloy phase is prepared by blending powders of the constituent elements of the desired oxide superconductor and silver. The blended powders are copper-deficient or may contain no copper, if desired. Silver comprises 70-80% vol of the blend. The resulting blend is then mechanically alloyed, as described, for example, in US Patent Nos. 4,962,084 and 5,034,373, herein incorporated by reference. The mechanically alloyed powder is fed into an extrusion die through which a plurality of copper wires is concurrently introduced. A composite wire is obtained having a primary alloy phase consisting of silver and copper-deficient, constituent elements of the desired oxide superconductor and a secondary phase of copper wires ranning coaxially along the length of the extruded wire. Example 2_. A primary alloy phase is prepared, as described in Example 1 or using any conventional alloying technique, containing the elements Pb, Bi, Sr, Ca, Cu and Ag in the atomic ratio of 0.34 : 1.74 : 1.92 : 2.05 : 3.07 : 0 to 0.34 : 1.74 : 1.92 : 2.05 : 3.07 : 14.5. The alloy is heated to a temperature in the range of 450 to 700 "C in an inert atmosphere for 10 seconds or more to obtain a composite having copper regions disposed within the primary alloy phase. Example ^ The process diagram in Fig. 5 illustrates the main elements of the process. The process begins with the mechanical alloying of metallic elements to form a homogeneous copper-deficient alloy powder 41 (primary alloy phase), as described above for Example 1. The alloy powder 41 is packed into a silver or copper can 42 (secondary phase) containing either a copper foil lining or strands of copper wire 44. Fig. 5 illustrates the process for an embodiment utilizing copper wires 44 coaxially aligned within the silver or copper can 42. The silver or copper can 42 containing the powder alloy 41 and copper 44 is then sealed and extruded into a hexagonal rod 46. Cut pieces of the rod are stacked into a multi- rod bundle that is again packed into a silver or copper can 47 and extruded into a hexagonal rod. This process is repeated several times. In the final step, the can is extruded through a die 48 into a rectangular or round wire 46. Tapes between 200 and 3,000,000 filaments can be readily made using multiple stacking. Fig. 6 is an optical photomicrograph showing a cross-section of a multifilament wire 50 of the invention containing a primary alloy phase 52, a secondary copper phase 53 and a silver matrix material 54. Another embodiment of the present invention includes an oxide composite which includes a primary oxide phase comprised of fully dense suboxides of a desired oxide superconductor and a secondary silver phase disposed within the primary oxide phase. In another embodiment, the composite includes a primary oxide phase and a secondary silver phase and a matrix material for supporting the primary oxide phase and secondary silver phase disposed therein. Fig. 7 is an optical photomicrograph of a cross-section of a wire 60 of this embodiment of the present invention having a primary oxide phase 62, a secondary silver phase 64 and a matrix material (here, silver) 66. Note that the composite is fully dense, that is, there are no visible porosity (gaps or voids) at grain boundaries (within phases) or at interfaces between phases. These microstructures are typical of composites prepared from oxidation of a precursor alloy and can be compared with composites prepared from powders of the sub-oxides, which are typically much less dense and have porosity within the phase and at phase interfaces. The embodiment of Fig. 7 can be prepared as shown in Examples 4 and 5. Example 4 ^ A multifilament composite is prepared according to Example 3. The composite is then oxidized under the conditions of 320 to 420 * C with oxygen pressures of 800 to 2000 psi (O 2 ) for 200 to 600 h, in particular, 420 * C for more than 200 h at 1600 psi (O 2 ). The process occurs as follows. Calcium and strontium are quickly oxidized to the corresponding metal oxides; bismuth and lead do not alloy significantly with copper and hence there is no significant migration of Ca, Pb, Sr or Bi into the copper secondary phase. The copper migrates towards the higher oxygen activity, i.e. , out of the secondary phase and into the region of the primary alloy phase,thereby creating a void in the secondary phase. The void is displaced by silver due to the very high surface energy associated with the void. Example 5__ A multifilament composite is prepared according to Example 3, with the following changes. The alloy powder 41 is alloyed with sufficient amount of copper to form a desired oxide superconductor and the copper wires 42 are replaced by silver wires. The assembled composite is oxidized under conditions sufficient to oxidize the alloy powder to suboxides. Typical oxidation conditions are 320 'C to 420 * C at 800 to 2000 psi O 2 for 200 to 600 h. The precursor composite and the oxide composite of the present invention have several advantages. The composites exhibit reduced diffusion of copper into the matrix material upon oxidation of the composites to form an oxide superconductor. This can be demonstrated by comparison of the oxide composites in Figs. 7 and 8. Fig. 7 represents an oxide composite which has been prepared from the oxidation according to the method of the invention of a precursor composite having a primary oxide phase supporting copper wires (secondary phase) in a silver matrix. Fig. 8 represents an oxide composite which has been prepared from a precursor composite without a secondary copper phase (copper is present in the primary alloy phase). Intrusion of the metal oxides, typically mostly copper oxide, although oxides of other metals can also form, into the silver matrix occurs to varying extent in both composites. However, metal oxide intrusion (which appears as fine tendrils 68 of metal oxide) is severely restricted in Fig. 7 compared to the two-dimensional front 71 of intruded metal oxide shown in Fig. 8. The two-dimensional "halo" of metal oxide surrounding the sub-oxide phase of the composite represents a much larger proportion of the constituent elements that the linear tendrils of Fig. 7. The oxide intrusion into the silver matrix remains upon further heat treatment to form an oxide superconductor. Further, the concentration of silver in the secondary phase of the oxide composites of the present invention provides an interface capable of preferential growth of the oxide superconductor. There has been suggestions in the prior art that silver/oxide interfaces promote the oriented growth of oxide superconductor grains, leading to texturing and improved critical transport characteristics (see, Feng et al. ia Appl. Phys. Lett. , 1993, hereby incorporated by reference). The compositions disclosed above can be used in the preparation of oxide superconducting composites. The composition may include the superconducting oxide phase including, but not limited to Bi 2 --Pb 4 Sr 2 Ca 2 Cu 3+z O x , where x is sufficient to provide T c > 90 K, and 0 < y ≤ 0.6 and 0 ≤ z < 1.0, Bi 2y Pb 4 Sr 2 Ca 2 Cu 3+z O x , where x is sufficient to provide T c > 77 K, and 0 < y ≤ 0.6 and 0 ≤ z ≤ 0.3 and A n Ba 2n Cu a(3n+1) O x , where A = (Reι. y Ca y ), n = (1, 2, 3 ... oo), a = (1.0 - 1.3) and 0 < y < .2 and x is sufficient to provide T c > 65 K. Re here is a rare earth or yttrium. It may be preferable to have excess copper in the oxide superconductor in the range of 10 to 30% excess based on the ideal copper cation stoichiometry of the oxide superconductor. Such compositions include, but are not limited to compositions having the following cation stoichiometries: Bi 2 . y Pb y Sr 2 Ca 2 Cu 35 . 37 ; Bi^Ph y S^Ca^.-,.^; and A 1 Ba 2 Cu 4 . 6 . 52 . The preparation of oxide superconducting composites using the composites and oxide composites of the present invention are described in Examples 6-10. Example 6. A primary alloy phase is prepared by blending the elements of Pb, Bi, Sr, Ca and Ag in the atomic ratio of 0.34 : 1.74 : 1.92 : 2.05 : 14.5. The resulting blend is then mechanically alloyed, as described, for example, in US Patent No. 5,034,373, herein incorporated by reference. Thirty three fine copper wires (0.5 mm in diameter) are coaxially arranged within a silver billet 0.615 inches OD x .552 inches ID x. 5.0 inches long (matrix material) and the alloyed powder containing Pb, Bi, Sr, Ca and Ag is packed into the billet and around the copper wires. The material within the silver billet has a final composition of 0.34 Pb : 1.74 Bi : 1.92 Sr : 2.05 Ca : 3.07 Cu: 14.5 Ag. The silver billet was extruded through a die to provide a composite wire having a silver matrix and plurality of primary alloy phase regions, each region supporting a secondary phase of copper wires. A plurality of wires prepared as described in the preceding paragraph are bundled together and coextruded to obtain a multifilament wire. An optical photomicrograph of a typical multifilament wire prepared from the composite of the present invention is shown in Fig. 6. The multifilament wire is then oxidized at 420 "C for 288 h in 100 atm oxygen. Under these oxidizing conditions, the copper diffuses out of the secondary phase and into the primary alloy phase. Concurrently, the silver of the primary alloy phase migrates towards the regions of the secondary, phase to be concentrated in the secondary phase, thereby forming a metal oxide/silver composite. The oxide composite sample is further heat treated in 0.075 atm O 2 for a total time of 7 to 15 h at a temperature of 780 * C with intermediate deformations through rolling of 75% to 82% strain. The temperature was increased to 831 * C for a further 10 to 60 h in 0.075 atm O 2 and then deformed a further 16 to 20% strain for a total strain in the range of 80 to 85%. A final heat treatment at 830 * C for 60 h and then 811 'C for 180 h provides a Bi 2 . y Pb y Sr 2 Ca 2 Cu 3 O x oxide superconductor, where y is O ≤ y < 0.6. The critical current densities of samples prepared according to this method are given in Table 1. Example 7. A precursor alloy is prepared and processed in the manner described in Example 6 above, with the following exception. Copper is mechanically alloyed with the other constituent elements of the oxide superconductor. No copper secondary phase is used; this is a conventional precursor alloy. The elements of Pb, Bi, Sr, Ca, Cu and Ag in the atomic ratio of 0.34 : 1.74 : 1.92 : 2.05 : 3.07 : 14.5. The resulting blend is then mechanically alloyed. The precursor alloy is processed as described in Example 3. A Bi 2 . y Pb y Sr 2 Ca 2 Cu 3 O x oxide superconductor is obtained, where 0 < y < 0.6. The critical current densities of wire samples prepared according to this method are given in Table 1. Example 8. A precursor alloy is prepared by blending the elements of Pb, Bi, Sr, Ca and Ag in the atomic ratio of 0.34 : 1.74 : 1.92 : 2.05 : 14.5. Thirty three fine copper wires (0.5 mm in diameter) are coaxially arranged within a silver billet the alloyed powder containing Pb, Bi, Sr, Ca and Ag is packed into the billet and around the copper wires. The material within the silver billet has a final composition of 0.34 Pb : 1.74 Bi : 1.92 Sr : 2.05 Ca : 3.70 Cu: 14.5 Ag, providing excess copper to the composite. The precursor composite is processed and thermally treated as described in Example 6. The critical current densities of wire samples prepared according to this method are given in Table 1. Example 9. A precursor alloy is prepared by blending the elements of Pb, Bi, Sr, Ca, Cu and Ag in the atomic ratio of 0.34 : 1.74 : 1.92 : 2.05 : 3.52 : 14.5. The alloyed powder containing Pb, Bi, Sr, Ca, Cu and Ag is packed into a silver billet. The material within the silver billet has a final composition of 0.34 Pb : 1.74 Bi : 1.92 Sr : 2.05 Ca : 3.52 Cu: 14.5 Ag. The precursor composite is processed and thermally treated as described in Example 6. The critical current densities of wire samples prepared according to this method are given in Table 1. Table 1. Comparative Critical Current Densities for Examples 6-9 No. Total t (hour) 3 Access 2° J c (A/cm 2 ) strain Cu Cu (%) 1 2 3 Phase 6 85 20 60 120 No Yes 7160 6 85 40 60 120 No Yes 5410 7 85 20 60 120 No No 3830 8 80 20 60 180 Yes Yes 8770 8 85 20 60 120 Yes Yes 9160 9 80 20 60 180 Yes No 5100 9 85 20 60 180 Yes No 6440 7 1 = heat treatment at 831 * C followed by deformation; 2 = heat treatment at 831 "C 3 = heat treatment at 811 * C followed. Table 1 clearly shows that excess copper levels in the oxide superconductor composite, and the presence of a secondary copper phase contribute to higher critical current densities in the sample. What is further clear is that these process parameters are process independent in that the parameters, alone and in combination, improve Jc. Further visual examination of the oxide superconducting samples prepared from the composite of the present invention show reduced segregation of copper into the silver matrix. This is supported by comparison of oxide composites of Figs. 7 and 8. The nature of the deformation and anneal conditions can also have an effect on the final superconducting properties of the composite as shown in the example below. Example 10. A multifilamentary composite wire is prepared as follows. A precursor alloy is prepared by blending the elements of Pb, Bi, Sr, Ca and Ag in the atomic ratio of 0.34 : 1.74 : 1.92 : 2.05 : 14.5. The resulting blend is then mechanically alloyed, as described, in Examples above. The alloyed powder containing Pb, Bi, Sr, Ca and Ag is packed into the billet and around the copper wires. The material within the silver billet has a final composition of 0.34 Pb : 1.74 Bi : 1.92 Sr : 2.05 Ca : 3.80 Cu: 14.5 Ag. The silver billet was extruded through a die to provide a composite wire with a hexagonal cross-section. A plurality of wires thus prepared are bundled together and coextruded to obtain a multifilament wire and extruded to provide a precursor tape or wire. Following precursor tape manufacture, the alloy filaments are oxidized to form fine grained, dispersed sub-oxide phases by diffusing oxygen through the silver matrix, exploiting silver's high permeability to atomic oxygen. Following oxidation, the precursor oxide filaments are reacted by thermal treatment(s) to form highly aspected, superconducting oxide grains with the c- directions orthogonal to their large surfaces. The reaction path for Bi 2 . y Pb y Sr 2 Ca 2 Cu 3+z O x (Bi-2223) involves the well known initial formation of Bi-2212 and "0011" reactant (compositionally CaCuO 2 ) from the suboxide phases, reaction (1), followed by conversion to Bi-2223, reactions (2), via an intercalation mechanism that reproduces the texture of the assemblage in the Bi-2223 phase assemblage formed. (2-y)BiOi. 5 + PbO + 2SrO + 2CaO + 3CuO =» Bi 2 . y Pb y Sr 2 CaCu 2 O x + CaCuO 2 Bi 2 . y Pb y Sr 2 CaCu 2 O x + CaCuO 2 => Bi 2 . 5 Pb y Sr 2 Ca 2 Cu 3 O x Both the Bi-2212 and Bi-2223 phases are textured by deformation processes such as rolling or uniaxial pressing. The oxide composite sample is further heat treated in 0.075 atm O 2 for a total time of 7 to 15 h at a temperature of 780° C with intermediate deformations through rolling of 75% to 85% strain. The temperature was increased to 830 'C for a further 20 h in 0.075 atm O 2 and then deformed a further 16 to 25% strain and heated treated for 60 hours at 830 * C in 7.5% O 2 and then finally heat treated at 811 * C for 180 h. The deformation used to texture the superconducting phase also fractures the oxides into discrete particles. The superconducting phase is therefore sintered by a final thermal treatment to form the interconnected structure inside each filament required for supercurrent transport through the multifilament composite. The oxide Jc dependencies on text strain are illustrated in Figure 9, for three thermal processing variations. It is evident that Jc typically increases with increasing texturing strain to a maximum in the range of 82% to 89% strain, followed by a rapid decrease. The increase in Jc as strain increases is due to improved texture, and the decrease is due to damage in the filaments from strain localization. Thermal process variations improve Jc by enhancing texturing strain efficacy as seen by the overall upward shift of the Jc v. strain relations for three thermal process variations. The optimal texturing strains in the range of 82 to 89% are small in comparison to the total strain (>99%) required to fabricate a multifilament tape. These total texturing strains are achieved by one or more deformation step wherein a deformation step is one or more applications of force to be the material between thermal treatments. The bulk of the deformation required for making high Jc multifilament wires in the process can therefore be done with the precursor filaments in the ductile metallic state, rather than in the more brittle oxide state. The transport properties attained with high-filament-count precursor composite processed tapes are presented in Table 2. Table 2. Transport properties of 259 filament, Bi-2223 - silver compos tapes made from precursors composites Length Temperature Oxide Jc 1 (m) (K) (A/cm 2 ) 0.03 77 17,700 0.03 4.2 71,000 8 77 8,000 ■ Jc is measured with the lμV/cm criterion (DC) by the four point probe method. The 77 K short length oxide Jc level in Table 2 exceeds the best levels reported for any other filament count process. Furthermore, these tapes were textured by scalable processes such as rolling, allowing extension of the process to long lengths. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification an examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims. What is claimed is:

Claims: 1. A method for preparing an unsegregated metal oxide/silver composite, comprising the steps of: preparing a precursor alloy, the precursor alloy comprising silver and precursor elements in a stoichiometry sufficient to provide a desired metal oxide; and oxidizing the precursor alloy under a condition of high oxygen activity selected to permit diffusion of oxygen into silver while significantly restricting the diffusion of the precursor elements into silver, such that oxidation of the precursor elements to the desired metal oxide occurs before diffusion of the precursor elements into silver. 2. A method for preparing an unsegregated oxide superconductor/silver composite, comprising the steps of: preparing a precursor alloy, the precursor alloy comprising silver and precursor elements in a stoichiometry sufficient to provide a desired oxide superconductor; oxidizing the precursor alloy under a condition of high oxygen activity selected to permit diffusion of oxygen into silver while significantly restricting the diffusion of the precursor elements into silver, such that oxidation of the precursor elements to a corresponding metal oxide occurs before diffusion of the precursor elements into silver; and heating the corresponding metal oxide at a temperature and pressure selected to convert the metal oxide into the desired oxide superconductor. 3. The method of claim 1 or 2, wherein high oxygen activity is obtained using high oxygen pressure. 4. The method of claim 1 or 2, wherein high oxygen activity is obtained using an oxygen-releasing gas. 5. The method of claim 1 or 2, wherein high oxygen activity is obtained by applying electromagnetic radiation. 6. The method of claim 1, 2 or 3, wherein the precursor alloy is oxidized at a temperature in the range of 200-450 °C. 7. The method of claim 1, 2 or 3 wherein the precursor alloy is oxidized at a temperature in the range of 300-430 °C. 8. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure above ambient pressure. 9. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure in the range of 15-3000 psi. 10. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure in the range of 800-3000 psi. 11. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure in the range of 1200-1800 psi. 12. The method of claim 1 or 2, wherein the precursor alloy is oxidized at a temperature in the range of 200 to 450 °C and at an oxygen pressure in the range of 15 to 3000 psi. 13. The method of claim 2, wherein the oxide superconductor comprises YBa 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K. 14. The method of claim 2, wherein the oxide superconductor comprises YBa 2 Cu 4 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K. 15. The method of claim 2, wherein the oxide superconductor comprises Bi 2 Sr 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K. 16. The method of claim 2, wherein the oxide superconductor comprises (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K. 17. The method of claim 2, wherein the oxide superconductor comprises (Ba,Pb) 2 Sr 2 Ca 1 Cu 2 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K. 18. The method of claim 2, wherein the oxide superconductor comprises Hg 1 Ba 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K 19. The method of claim 2, wherein the oxide superconductor comprises Hg ! Ba 2 CaιCu 2 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K. 20. The method of claim 2, wherein the oxide superconductor comprises HgjBa 2 CuιO x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 77K. 21. The method of claim 2, wherein the oxide superconductor comprises (Tl, Pb) 1 Sr 2 Ca 1 Cu 2 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K. 22. The method of claim 2, wherein the oxide superconductor comprises (Tl,Pb)ιSr 2 Ca 2 Cu 3 O x , wherein x is sufficient to provide an oxide superconductor having a T c greater than 100K. 23. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure in the range of 15-3000 psi and a total gas pressure of 16- 60,000 psi with a second gas used to dilute the oxygen for enhanced total pressure above the desired oxygen pressure. 24. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure in the range of 800-3000 psi and a total gas pressure of 801- 60,000 psi with a second gas used to dilute the oxygen for enhanced total pressure above the desired oxygen pressure. 25. The method of claim 1 or 2, wherein the precursor alloy is oxidized at an oxygen pressure in the range of 1200-1800 psi and a total gas pressure of 1201-60,000 psi with a second gas used to dilute the oxygen for enhanced total pressure above the desired oxygen pressure. 26. The method of claim 1 or 2, wherein the precursor alloy is prepared by a method comprising the steps of: introducing finely divided precursor elements into a silver can; extruding the precursor element-containing can into a first wire; grouping a plurality of extruded first wires to form a bundle of wires; and co-extruding the bundle of wires to form a second wire having a plurality of precursor element filaments therein. 27. A method for preparing a metal oxide/silver composite, comprising the steps of: preparing a composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor and silver and a secondary phase comprising copper, the secondary phase supported by the primary alloy phase; oxidizing the composite under conditions sufficient to oxidize the constituent elements to at least one metal oxide and under conditions which promote the diffusion of the silver of the primary alloy phase into the region of the secondary phase and which promote the diffusion of the copper of the secondary phase into the region of the primary alloy phase, so that a substantially pure silver phase occupying substantially the secondary phase is formed. 28. The method of claim 1 or 2, wherein the step of preparing a precursor alloy further comprises of: providing a composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor and silver and a secondary phase comprising copper, the secondary phase supported by the primary alloy phase. 29. The method of claim 1 or 2, wherein the step of preparing a precursor alloy further comprises of: prividing a precursor composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor and silver, a secondary phase comprising copper, the secondary phase supported by the primary alloy phase and silver as a matrix for supporting the primary alloy phase and the secondary phase. 30. The method of claim 27, wherein the condition which promote diffusion of the silver into the region of the secondary phase and diffusion of the copper into the region of the primary alloy phase comprises oxidation in an oxygen pressure of 800 to 3000 psi with a total pressure of 801 to 60,000 psi. 31. A method for making a copper phase-separated composite comprising: preparing an alloy comprising copper; and heating the alloy to a temperature in the range of 450 to 700 °C in an inert atmosphere for 10 or more seconds sufficient to phase separate the copper. 32. A method for preparing an oxide superconductor, comprising the steps of: preparing a composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor and silver and a secondary phase comprising copper, the secondary phase supported by the primary alloy phase; oxidizing the composite under conditions sufficient to form an oxide superconductor. 33. A method for preparing an oxide superconductor composite, comprising the steps of: preparing a precursor composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor and silver, a secondary phase comprising copper, the secondary phase supported by the primary alloy phase and a matrix for supporting the primary alloy phase and the secondary phase therein; and oxidizing the precursor composite under conditions sufficient to form an oxide superconductor composite. 34. The method of claim 32 or 33, further comprising the step of: texturing the oxide superconductor composite by subjecting the oxide superconducting composite to at least one deformation and anneal step, wherein the total deformation in the range of 82% to 89% strain and wherein the anneal step comprises conditions sufficient to form the desired oxide superconductor. 35. The method of claim 34, further comprising the step of: oxidizing the precursor composite under conditions sufficient to oxidize the constituent elements to at least one metal oxide and under conditions which promote the diffusion of the silver of the primary alloy phase into the region of the secondary phase and which promote the diffusion of the copper of the secondary phase into the region of the primary alloy phase, to form an intermediate oxide composite where a substantially pure silver substantially occupies the secondary phase; and further oxidizing the intermediate oxide composite to form an oxide superconductor composite. 36. The method of claim 32 or 33, wherein the primary alloy phase of the precursor composite comprises a portion of the copper required for cation stoichiometry. 37. The method of claim 32 or 33, wherein the secondary phase additionally comprises silver. 38. The method of claim 32 or 33, wherein the constituent elements of the primary alloy phase and copper of the secondary phase are, in combination, present in an amount sufficient to form the desired oxide superconductor. 39. The method of claim 32 or 33, wherein the secondary phase of copper is introduced into the precursor alloy composite in a form selected from a group consisting of particle, foil, wire and rod. 40. The method of preparing a textured oxide superconductor, comprising the steps of: providing a precursor containing the constituent elements of a desired oxide superconductor; oxidizing the precursor to form an oxide phase capable of being reacted to form the desired oxide superconductor; and texturing the oxide phase by at least one deformation and anneal step to form the desired oxide superconductor, wherein the total strain is in the range of 82% to 89% and wherein the anneal step comprises conditions sufficient to form the desired oxide superconductor. 41. The method of claim 40, wherein the total strain is accomplished in 5 to 30 deformation steps. 42. The method of claim 40, wherein the anneal step comprises heating at a temperature in the range of 700 to 860 °C at a pressure in the range of 0.9 to 1.1 atm with gas comprising 7.5% to 22% oxygen and the balance nitrogen. 43. A method for preparing an oxide superconductor composite comprising the steps of: preparing a precursor composite comprising a primary alloy phase comprising the constituent elements of a desired oxide superconductor present in an amount sufficient to form the desired oxide superconductor and a secondary phase comprising silver, the secondary phase disposed within the primary alloy phase; and oxidizing the precursor composite under conditions sufficient to form an oxide superconductor. 44. A metal oxide-silver composite prepared according to the method of claim 1 or 28. 45. The composite of claim 44, further characterized in that the metal oxide-silver composite has a microstructure in which a cross-section transverse to a longest dimension consists of the silver phase and clearly defined metal oxide regions having a density of metal oxide regions of at least 10,000 regions/cm 2 . 46. The composite of claim 44, further characterized in that the silver phase contains no metal oxide beyond a distance of three microns from the interface of the desired metal oxide phase and silver. 47. An oxide superconductor-silver composite prepared according to the method of claim 2 or 29. 48. The composite of claim 47, further characterized in that the silver matrix contains no metal oxide beyond a distance of three microns from the interface of a corresponding oxide superconductor phase and silver. 49. The composite of claim 47, further characterized in that the oxide superconductor-silver composite has a microstructure in which a cross-section transverse to a longest dimension consists of the silver matrix and a clearly defined oxide superconductor regions having a density of oxide regions of at least 10,000 regions/cm 2 . 50. A metal oxide-silver composite, comprising: a desired metal oxide phase and silver phase, the desired metal oxide phase having an unsegregated microstructure, characterized in that a metal oxide phase resulting from the oxidation of a segregated precursor element of the metal oxide phase is no more than a distance of three microns from the interface of the desired metal oxide phase and the silver phase. 51. The composite of claim 50, wherein the metal oxide phases further comprises of secondary silver phase, supported by the primary metal oxide phase. 52. An oxide superconductor-silver composite, comprising: a superconducting oxide phase and silver phase, the oxide superconductor phase having an unsegregated microstructure, characterized in that a metal oxide phase resulting from the oxidation of a segregated precursor element of the oxide superconductor phase is no more than a distance of three microns from the interface of the corresponding oxide superconductor phase and silver phase. 53. An oxide superconductor-silver composite, comprising: a superconducting oxide phase and silver phase, the oxide superconductor phase having an unsegregated microstructure, characterized in that a metal oxide phase resulting from the oxidation of a segregated precursor component of the oxide superconductor phase is observed at distances no greater than 20% of the thickness and width of the oxide superconductor phase. 54. The composite of claim 52 or 53 wherein the superconducting oxide phase further comprises of secondary silver phase, supported by the primary metal oxide phase. 55. An oxide superconductor-silver composite, characterized in that the oxide superconductor-silver composite has a microstructure in which a cross- section transverse to a longest dimension consists of the silver matrix and a clearly defined oxide superconductor regions having a density of oxide regions of at least 10,000 regions/cm 2 . 56. A composite, comprising: a primary alloy phase comprising the constituent elements of a desired oxide superconductor; and a secondary phase comprising copper, the secondary phase supported by the primary alloy phase. 57. A composite, comprising: a primary alloy phase comprising the constituent elements of a desired oxide superconductor; a secondary phase comprising copper, the secondary phase supported by the primary alloy phase; and a matrix for supporting the primary alloy phase and the secondary phase therein. 58. The composite of claim 56 or 57 wherein the constituent elements of the primary phase and the copper of the secondary phase are, in combination, present in an amount sufficient to form the desired oxide superconductor. 59. The composite of claim 56 or 57, wherein the primary alloy phase comprises a noble metal. 60. The composite of claim 56 or 57, wherein the primary alloy phase and the secondary phase are constituted and arranged to promote exchange of elements between phases. 61. The composite of claim 60, wherein the primary phase comprises silver and wherein the exchange comprises movement of silver from the primary alloy phase into the region of the secondary phase and movement of copper from the secondary phase into the region of the primary alloy phase. 62. The composite of claim 59, wherein said noble metal comprises silver. 63. The composite of claim 56 or 57, wherein the secondary phases comprises a noble metal. 64. The composite of claim 63, wherein said noble metal comprises silver. 65. The composite of claim 56 or 57, wherein copper is present in stoichiometric excess in the range of 10% to 30% with respect to the ideal copper cation stoichiometry of the desired oxide superconductor. 66. The composite of claim 56 or 57, wherein the ideal formula of the desired oxide is Bi2-yPbySr2Ca2Cu3 +zOx, where 0 < y < 0.6; where 0 < z ≤ 1.0 and x is sufficient to provide Tc > 90 . 67. The composite of claim 66, wherein 0.5 < z < 1.0. 68. The composite of claim 56 or 57, wherein the ideal formula of the desired oxide is Bi2-yPbySr2CalCu2+z Ox, where 0 < y < 0.6; where 0 < z < 0.7; and x is sufficient to provide Tc > 77 K. 69. The composite of claim 68, wherein 0.3 < z < 0.7. 70. The composite of claim 56 or 57, wherein the ideal formula of the desired oxide is AnBa2nCua(3n+l)Ox, where A = (Rel-y,Cay) and 0 < y < 0.2; n ranges from 1 to oo ; 1.00 < a< 1.3; and x is sufficient to provide Tc > 65 K. 71. The composite of claim 70, wherein n = land 1.15 < a ≤ 1.3. 72. The composition of claim 56 or 57, wherein the secondary phase is disposed within the primary alloy phase. 73. The composite of claim 56 or 57, wherein the secondary phase contactingly surrounds at least a portion of the primary alloy phase. 74. The composite of claim 56 or 57, wherein the copper-containing secondary phase is disposed within the primary alloy phase as a plurality of discrete components. 75. The composite of claim 56 or 57 wherein the secondary phase comprises a plurality of extended components coaxially aligned within the primary alloy phase. 76. The composite of claim 56 or 57, wherein the secondary phase is selected from a group consisting of wire, rod, particle and foil. 77. The composite of claim 76, wherein the secondary phase comprises 1 to 1000 wires. 78. The composite of claim 56 or 57, wherein substantially all of the copper of the composite is present in the secondary phase. 79. The composite of claim 56 or 57, wherein a portion of the copper constituting the desired oxide superconductor is present in the secondary phase, the balance comprising the primary alloy phase. 80. An oxide composite, comprising a primary oxide phase comprising fully dense sub-oxides of a desired oxide superconductor; and a secondary silver phase, disposed within the primary oxide phase. 81. An oxide composite, comprising a primary oxide phase comprising one or more oxides selected from the group consisting of simple, binary and higher oxides of the constituent elements of a desired oxide superconductor; a secondary silver phase disposed within the primary alloy phase; and a matrix for supporting a primary alloy phase and the secondary silver phase therein. 82. The oxide composite of claim 81, wherein the composite is characterized in that the oxides of the constituent elements intrude linearly into the matrix. 83. The composite of claim 80 or 81, wherein the oxides of the primary oxide phase are present in an amount sufficient to form a desired oxide superconductor. 84. The composite of claim 80 or 81, wherein oxides of copper are present having a stoichiometric excess of copper in the range of 10% to 30% with respect to the ideal copper cation stoichiometry of the desired oxide superconductor. 85 The composite of claim 80 or 81, wherein the ideal formula of the desired oxide is Bi 2 . y PbySr 2 Ca 2 Cu 3+z O x , where 0 < y < 0.6; where 0 < z < 1.0; and x is sufficient to provide Tc > 90 . 86. The composite of claim 85, wherein 0.5 ≤ z < 1.0. 87. The composite of claim 80 or 81, wherein the ideal formula of the desired oxide is Bi 2 - y Pb y Sr 2 CaιCu 2+z O x , where 0 < y < 0.6; where 0 ≤ z < 0.7; and x is sufficient to provide T c > 77 K. 88. he composite of claim 87, wherein 0.3 < z < 0.7. 89. The composite of claim 80 or 81, wherein the ideal formula of the desired oxide is A n Ba 2n Cu a(3n+1) O x , where A = (Re^.Ca y ) and 0 < y < 0.2; n ranges from 1 to oo ; 1.00 < a< 1.3; and x is sufficient to provide T c > 65 K. 90. The composite of claim 89, wherein n = 1 and 1.15 < a < 1.3. 91. An oxide superconducting composite, comprising: a primary fully dense oxide superconducting phase, the oxide superconducting phase comprising a stoichiometric excess of copper in the range of 10% to 30% with respect to ideal copper cation stoichiometry; and a matrix for supporting the primary oxide superconducting phase disposed therein, the composite characterized in that simple oxides of the constituent elements intrude linearly into the matrix. 92. The composite of claim 91 further comprising of a secondary silver phase supported by the primary oxide superconducting phase.

Inventors: CRAVEN CHRISTOPHER A, MASUR LAWRENCE J, OTTO ALEXANDER, PODTBURG ERIC R, SANDHAGE KENNETH H, SCHREIBER JEFFREY D, CRAVEN, CHRISTOPHER, A., MASUR, LAWRENCE, J., OTTO, ALEXANDER, PODTBURG, ERIC, R., SANDHAGE, KENNETH, H., SCHREIBER, JEFFREY, D.
Applicants: AMERICAN SUPERCONDUCTOR CORP, AMERICAN SUPERCONDUCTOR CORPORATION
Application year: 1994
Main classification: C04B35
IPC codes: C01G1, C01G3, C01G13, C01G15, C01G29, C04B35, H01L39